Release 10 of the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) specification includes several features related to downlink (DL) and uplink (UL) MIMO, relays, bandwidth extension via carrier aggregation and enhanced inter-cell interference coordination (eICIC). In DL MIMO, in order to meet the peak spectral efficiency requirements of up to 30 bit/s/Hz, Release 10 extends Release 8/9 DL MIMO features by providing support for up to 8 stream transmissions, and hence up to 8×8 MIMO. Furthermore, enhanced support of multi-user (MU) MIMO is enabled in Release 10 and seamless switching between single- and multi-user operations is supported.
Release 10 further provides a double codebook for 8 transmit (Tx) antennas. The double codebook for 8 Tx antennas is based on a modular design (or multi-granular), combining two feedback components from distinct codebooks: one feedback component represents the long-term (e.g. wideband) radio channel properties while the other one targets the short term (e.g. frequency selective) channel properties.
Non-uniform network deployments, also known as heterogeneous networks (HetNet), are potential scenarios considered in Long Term Evolution Release 11 for DL MIMO and coordinated multi-point transmission (COMP). HetNet was considered during Release 10 within the context of enhanced inter-cell interference coordination/cancellation (eICIC) discussions, but Release 10 focused on mainly macro cell and pico cells deployed inside the macro cell, hence only relatively light interaction between the macro cells and the pico cells in the form of time-domain resource partitioning with the exchange of scheduling information and patterns over the backhaul link. One difference between the macro cell and the remote radio head (RRH) lies in the utilized transmit powers, since the macro cell may operate in the range of 46/49 dBm in a 10/20 MHz carrier while the RRHs could operate for example with 30/37 dBm. The HetNet scenarios in Release 11 are considered for both coordinated multipoint transmission and for single cell MIMO enhancements. For CoMP operation, detailed simulation assumptions have been described in R1-111125, CoMP simulation assumptions, which are hereby incorporated by reference.
For example, a macro cell may consist of an array of antennas while a low power RRH may have one or an array of transmit antennas. Each antenna or array of antennas is understood to be a transmission point; hence the macro cell is a transmission point while the RRHs are also transmission points. The RRH and the macro cell are generally connected through optical fiber, hence the feedback delays and capacity over the connection are considered as ideal and unlimited in this case. The RRHs placement may be indoor or outdoor. A particular example may include an instance in which a macro cell is not present while RRHs are connected to a central unit which performs radio resource management (RRM). In a first example, it is assumed that each transmit point has its own physical cell identifier (cell ID) and in a second example it is assumed that all transmit points have the same cell ID. In both example cases there is a central unit which performs scheduling of the radio resources and is located for example at the macro cell. The RRHs, in this example, are arrays of antennas which are typically used in order to improve the spectral efficiency of the cell. Hence they can be also seen as simple radio frequency front ends pulled away from the macro cell and without RRM capability.
In the second example, where all transmit points have the same cell ID, the transmit points may be equipped with various numbers of transmit antennas. From a simulation assumption perspective 3GPP is currently assuming 2, 4 and 8 transmission points for the macro cell and 1, 2 and 4 transmission points for the low power RRHs. In the simulations being carried out in 3GPP, both co-polarized and cross-polarized types of antennas are considered with the restriction that the same type of antennas are used for all transmit points in a given configuration. However, specification-wise it is likely there will be no restriction in terms of number or type of transmit antennas, hence 8 transmission points could be considered also for RRHs.
In this example, mobile terminals are located in a cell formed by the macro cell and lie under the coverage of the RRHs. In a traditional macro cell only scenario, these mobile terminals would be configured to determine the number of transmit antennas existing at the macro cell and report the channel state information (CSI) based on the common reference symbol (CRS) or channel state information reference symbol (CSI-RS) ports. CSI-RS provides support for 1, 2 4, and 8 transmission points. CSI-RS parameters, like the periodicity and pattern, are signaled as mobile terminal-specific information. In this scenario the mobile terminal may hear the RRH (or several RRHs) and the macro cell. In such cases, the mobile terminal signals the specific antenna ports associated with the transmission points on which it performs CSI estimation. For example, if the mobile terminal hears two RRHs and the macro cell, it receives the CSI-RS patterns and parameters of these three transmit points for which to compute the CSI. Once channel estimation is performed, CSI feedback may be computed and reported to a central scheduling unit, such as the macro cell. Both single user (SU) and MU MIMO may be supported; hence the computed feedback for these three transmit points enables closed-loop MIMO operation.
However, currently mobile terminal feedback operates on a single-transmission point basis or single-cell feedback. Hence, with such feedback, the maximum supported rank in the system is limited by the maximum of the individually supported ranks for each of the transmission points. Therefore the mobile terminal would typically report the rank based on the maximum rank supported by one individual transmission point, whereas the rank could in fact be higher if more transmission points were used for transmission and considered in finding the optimal rank.